This invention relates generally to thermostatic type circuit breakers used to interrupt an electrical circuit under selected overload conditions and more particularly to improvements in means for calibrating such circuit breakers.
Trip free, ambient compensated circuit breakers are well known. An example of this type of circuit breaker is disclosed and claimed in U.S. Pat. No. 3,361,882, assigned to the assignee of the present invention, the subject matter of which is incorporated herein by this reference. As described, the circuit breaker of this type has a movable contact assembly mounting a movable electrical contact for movement between open and closed contact positions with a stationary electrical contact. A latching mechanism is provided for maintaining the movable electrical contact in the closed contacts position against the bias of a contacts opening spring. An overload trip assembly includes a thermostatic actuator in the form of an elongated, U-shaped bimetallic member having at one end the free ends of two legs fixedly mounted to a supporting member in the housing of the circuit breaker with the bight or junction of the two legs forming a second end of the bimetallic member. The bimetallic member forms a part of the circuit path through the circuit breaker and with a selected overload, such as current exceeding the rated current value for a certain length of time, the bimetallic member will deflect with the second end thereof transferring motion to a motion transfer plate which in turn moves a latch receiving catch of the latching mechanism away from a latch of the movable contact assembly to thereby enable the contact opening spring to move the movable electrical contact to the open contacts position.
As shown in the referenced patent, the thermostatic actuator is calibrated by means of an adjusting screw threaded through one wall with the end of the screw aligned with a second parallelly extending wall. Sufficient rotation of the screw will bend the second wall moving it against the elongated bimetallic member adjacent the first end to thereby move the second end of the bimetallic member toward an ambient compensation assembly which includes the latch receiving catch on the other side of the circuit breaker. Small changes in the position of the calibration screw result in amplified displacements at the top of the second end of the bimetallic member, e.g., approximately 2.5:1, in devices made similar to those shown in the patent. For example, a quarter turn of a #0-80 UNF thread calibration screw moves the second wall approximately 0.003inch which, in turn, moves the top of the second end of the bimetallic member approximately 0.007inch.
During operation, current passes from a load terminal through one leg of the U-shaped bimetallic member and out the other leg through a conductive strap and then to the contacts. The bimetallic member warms up due to I2R heating and then bends due to the different coefficients of expansion of the bimetal layers. At calibrated current loads and temperatures, the bimetallic member will deflect and push the motion transfer plate the required distance to trip the circuit breaker latch mechanism.
A variation of the calibration mechanism, not shown in the patent but in wide use, is the use of a calibration clip which has one wall which holds the screw and a second spaced apart wall portion which holds the first end,of the bimetallic member. The screw is provided with a head which is nested into the second wall portion of the clip so that if the screw is turned in too far it can then be turned backwards concomitantly with the second wall portion to allow additional calibration attempts. In the previously discussed calibration structure described in the patents, some additional calibration attempts are possible due to spring back of the second wall when the screw is turned out, limited by stress relief and the like. This variation is subject to the same type of amplified motion as in the calibration structure described in the patent.
It is an object of the invention to provide a calibration assembly which has less sensitivity and one which enables increased performance repeatability. Another object of the invention is the provision of a circuit breaker having a calibration assembly which has improved durability. Another object is the provision of a circuit breaker which overcomes the prior art limitations noted above.
Briefly, in accordance with the invention, the calibration screw for the bimetallic member, also referred to as trip arm hereinafter, is placed in line with the top or second end of the trip arm. This provides one-to-one displacement for the screw to trip arm significantly reducing calibration sensitivity and increasing performance repeatability. According to a first embodiment, the trip arm of the actuator assembly is generally U-shaped with the free ends of the legs forming a first end of the trip arm. The free end of one leg is fixedly connected, as by welding, to a first end of a pivotable calibration base with a piece of insulation material, such as Kapton tape disposed about the free end of the second leg and then received in a clip portion of the calibration base bent back on itself to capture the leg. The calibration base is pivotably mounted in the housing of the circuit breaker and has a slotted second end aligned with and generally coextensive with the second end of the U-shaped trip arm, i.e., the bight or junction of the two legs with the trip arm disposed intermediate the calibration base and the motion transfer member. The calibration screw head is formed with a circumferentially extending groove which receives and captures the slotted end of the second end of the calibration head. Rotation of the calibration screw will cause pivoting movement of the actuator assembly toward or away from the motion transfer plate adjacent thereto.
Upon application of power, current flows through a load terminal to a first pigtail or flexible conductor to a leg of the trip arm and exits the trip arm through a second pigtail and then flows to the contacts. The trip arm heats due to I2R heating to provide mechanical deflection to trip the circuit breaker upon a selected overload.
In a second embodiment, the trip arm, i.e., the bimetallic member, extends in a straight line and is attached as by being welded to the bottom of a pivotably mounted calibration base which also extends in a straight line and is generally coextensive with the trip arm. In this embodiment the calibration base is disposed intermediate to the trip arm and the motion transfer member with the second end of the trip arm provided with a slot which is received in a circumferentially extending groove in the head of the calibration screw. A wider slot is provided in the second end of the trip member providing clearance for the head of the calibration screw. As in the first embodiment, the calibration screw is positioned in line with the motion transfer member. Thus, rotation of the calibration screw will move the second end of the actuator assembly toward or away from the motion transfer member.
During operation, current flows through a load terminal to a first pigtail and into the first end of the trip arm, then through the length of the trip arm out through a second pigtail to the contacts. With both ends of the trip arm supported, the trip arm bows as it becomes heated due to the different coefficients of expansion of the thermostatic layers. Since the trip arm and base are permanently attached near the bottom pivot location at the first end of the assembly, the second end of the calibration base rotates toward the motion transfer member with the change in slope of the bowing trip arm.